The present invention relates to a solid-state imaging device and an electronic device.
An electronic device such as a digital video camera or a digital still camera includes a solid-state imaging device. For example, a CMOS (complementary metal oxide semiconductor) type image sensor is used as the solid-state imaging device.
The solid-state imaging device has a plurality of pixels arranged on a surface of a semiconductor substrate. A photoelectric conversion unit is provided in each pixel. The photoelectric conversion unit is, for example, a photodiode, and generates signal charges by sensing light coming via an externally attached optical system in a light sensing surface and by photoelectric-converting the light.
Among the solid-state imaging devices, each pixel of the CMOS type image sensor includes a readout circuit in addition to the photoelectric conversion unit. The readout circuit includes a plurality of transistors, reads out signal charges generated in the photoelectric conversion unit, and outputs the read-out signal charges to a signal line as an electric signal.
In the CMOS type image sensor, the photoelectric conversion unit reads out the signal charges for each pixel or for each row where a plurality of pixels is arranged. In this case, exposure time for accumulating the signal charges is difficult to match in all the pixels, and thus in some cases, a captured image is distorted. Particularly, if the motion of a subject is great, this defect is noticeably generated.
In order to prevent this defect from being generated, a “global exposure” is performed in which all the pixels start exposure at the same time and finish the exposure at the same time.
The “global exposure” is performed, for example, in a “mechanical shutter method” using a mechanical shutter. Specifically, all the pixels start exposure by opening the mechanical shutter, and finish the exposure by closing the mechanical shutter. However, in this “mechanical shutter method,” the size of a device is difficult to decrease since a mechanical light blocking unit is used. Also, since the speed of the driving operation of the mechanism is difficult to increase, the simultaneous exposure in all the pixels is hard to perform with high accuracy.
The “global exposure” is performed in a “global shutter method” in addition to the “mechanical shutter method.” Specifically, the “global exposure” is performed by simultaneously driving all the pixels through an electrical control without using the mechanical shutter. In the global shutter method, a size of a device can be easily reduced since the mechanical light blocking unit is not used. Further, the speed of the driving operation is easily increased, and the simultaneous exposure in all the pixels can be performed with high accuracy (for example, refer to Japanese Unexamined Patent Application Publications Nos. 2004-055590, 2009-268083 and 2004-140152).
Meanwhile, in the solid-state imaging device, there is a demand for an increase in the number of pixels along with the small size. In this case, since the size of one pixel is small, it is hard for each pixel to sense a sufficient amount of light, and thus it is not easy to improve the image quality of a captured image. For this reason, it is necessary for the solid-state imaging device to have high sensitivity.
In order to realize the high sensitivity, there has been proposed one where a chalcopyrite based compound semiconductor film such as a CuInGaSe2 film with high light absorption coefficient is used in the photoelectric conversion unit (for example, refer to Japanese Unexamined Patent Application Publications No. 2007-123720).
In addition, there has been proposed a “lamination type” where photoelectric conversion units for respective colors are laminated and disposed in a depth direction perpendicular to an imaging surface, instead of disposing the photoelectric conversion units which selectively sense light beams of respective colors in a direction along the imaging surface. In the “lamination type,” each pixel senses not only light of one color but also light of plural colors. For this reason, a light sensing surface is extensively formed and thus use efficiency of light can be improved, thereby improving sensitivity (for example, refer to Japanese Unexamined Patent Application Publications No. 2006-245088).
Further, there has been proposed a “rear surface illumination type” where a photoelectric conversion unit senses light which is incident from a rear surface opposite to a front surface in which circuits, wires and the like are provided, in a semiconductor substrate. In the “rear surface illumination type,” the circuits, the wires and the like, which block or reflect the incident light are not provided in the incident side, and thus sensitivity can be improved (for example, refer to Japanese Unexamined Patent Application Publications No. 2008-182142). In the “rear surface illumination type,” there has been proposed one where a control gate electrode is formed in the photoelectric conversion unit on a surface opposite to the light sensing surface, a potential is controlled by applying a voltage to the photoelectric conversion unit, and signal charges are efficiently transferred (for example, refer to Japanese Unexamined Patent Application Publications No. 2007-258684).
In addition, the inventors of the present invention have recognized that in the solid-state imaging device, light enters an accumulator which accumulates signal charges generated by the photoelectric conversion unit or a readout circuit which reads out the signal charges, which causes noise to be generated, and thus there is a problem in that the image quality of a captured image is deteriorated.
In order to prevent the generation of such a defect, a light blocking film may block light from entering the accumulator or the readout circuit.
However, if the light blocking film is formed between the photoelectric conversion unit and the accumulator or the readout circuit, the area of the light sensing surface becomes small since the aperture ratio is reduced, and thus sensitivity is lowered in some cases.
In addition, light is diffracted or scattered due to the light blocking film, the diffracted light or the scattered light enters the accumulator to generate noise, and thus there are cases where the image quality of a captured image is deteriorated.
In the case of the “rear surface illumination type” solid-state imaging device, the accumulator or the readout circuit is formed on the front surface side opposite to the rear surface side which senses light in the substrate, but, in some cases, the above-described defects are generated since the substrate is thin for reading out the signal charges.
FIG. 60 is a cross-sectional view illustrating the “rear surface illumination type” solid-state imaging device.
FIG. 61 shows a simulation result of a form of light traveling in the “rear surface illumination type” solid-state imaging device. Here, a result of a case where light having a wavelength of 650 nm vertically enters the surface of the silicon substrate 101J (3 μm thick) is shown.
As shown in FIG. 60, in the “rear surface illumination type” solid-state imaging device, members such as on-chip lenses ML, color filters CF, and insulating films Z1, Z2 are provided on the rear surface side of the silicon substrate 101J. A wire layer 111 is provided on the front surface side of the silicon substrate 101J. The wire layer 111 is provided to cover a readout circuit (not shown) provided on the front surface side of the silicon substrate 101J.
In the “rear surface illumination type” solid-state imaging device, photodiodes (not shown) provided inside the silicon substrate 101J sense light passing through the respective portions such as the on-chip lenses ML and the color filters CF. Further, the readout circuit (not shown) provided on the front surface side of the silicon substrate 101J reads out signal charges from the photodiodes (not shown).
As shown in FIG. 61, in the “rear surface illumination type” solid-state imaging device, the light which enters the rear surface (the upper surface in FIG. 60) of the silicon substrate 101J through the respective portions such as the on-chip lenses ML and the color filters CF reaches the front surface (lower surface). Specifically, light passing through the red filter layer CFR reaches the front surface of the silicon substrate 101J more than light passing through the green filter layer CFG, and 28% of the light reaches the front surface.
As such, the inventors of the present invention have recognized that even in the “rear surface illumination type,” the light coming from the rear surface side is not blocked and reaches the front surface side on which the accumulator is provided, and thus there are cases where noise is generated and image quality of a captured image is deteriorated.
Particularly, in a case where imaging is performed in the “global shutter method,” since exposure in all the pixels is performed at the same time and then signal charges are temporarily accumulated in the accumulator, if light enters the accumulator, noise is notably generated.
Therefore, in the solid-state imaging device, there are cases where it is difficult for the small size and the improvement in the image quality of a captured image to be compatible.